This study evaluates the User eXperience (UX) of Polariscope, a mobile-first, web-based collaborative storytelling platform, designed to preserve, share, and co-create collective memories related to heritage and culturally significant events. Combining collaborative storytelling, AI-driven recommendations, thematic media galleries, and call-to-action challenges, the platform aims to stimulate interaction and user-generated narratives that may resonate collectively. A field trial with 30 participants was conducted during the ‘São Gonçalinho’ celebration in Aveiro, Portugal. Participants completed exploratory tasks (e.g., contributing to challenges, uploading content and creating stories), and, following the testing period, responded to an online questionnaire containing open- and closed-ended questions, as well as standardized usability and experience measures: the System Usability Scale (SUS), Self-Assessment Manikin (SAM), and AttrakDiff. Results indicated performance and interface issues, particularly among older users, and a strong preference for passive content consumption over active contribution, suggesting the need for strategies to foster participation. While half of the participants expressed interest in future use and found the features appealing, many reported difficulties understanding the platform’s structure and interaction model. Qualitative feedback generated targeted recommendations to enhance interface clarity, technical robustness, and engagement mechanisms. Overall, the findings reveal both the opportunities and limitations of digital cultural heritage platforms and highlight collaborative storytelling as a promising means of fostering user engagement and broader cultural participation.